While there is a smaller demand for electric power late at night than daytime, it is better to operate an electric power generator continuously, because doing so is more efficient. It is difficult to store generated electric power, and accordingly, electric power generation facilities have their capabilities set to generate electric power in accordance with a peak of demand for electric power. Under such a circumstance, it is well known that late-night power rate is lower than daytime power rate. It is desirable if households, companies, factories, and other electric power consumers can store midnight electric power in a reservoir type storage battery and use it in the daytime to achieve a reduced electricity bill and a leveled load.
Japanese Patent Laying-Open No. 2001-008380 (PTL 1) discloses a system allowing an electric vehicle and a residence to mutually transmit electric power and indicates leveling a demand for electric power.